1. Field of the Invention
This invention relates to a physical quantity measuring system in which an amount of a light output from a fiber Bragg grating (FBG) can be measured exactly in a vicinity of a center wavelength of an output channel of an arrayed waveguide grating (AWG).
2. Description of the Related Art
There has been a physical quantity measuring system in which a physical quantity (e.g., temperature, strain, etc.) is measured according to a change of a reflected center wavelength of a fiber Bragg grating (FBG) in an optical fiber. In this physical quantity measuring system, an arrayed waveguide grating (AWG) is used.
FIG. 5 shows a conventional physical quantity measuring system disclosed in patent document 1 (JP-B-3760649). In this physical quantity measuring system, several FBGs (i.e., FBG1, FBG2, FBG3, FBG4, and FBG5) are formed in an optical fiber to which a measuring light from a broadband light source is inputted. A physical quantity, in a position where each FBG is disposed, is measured by detecting a wavelength of a reflected light from each FBG.
In this physical quantity measuring system, a minute wavelength range of the reflected light is assigned to the several FBGs without an overlap, the reflected light from each FBG is inputted to an AWG which can divide a light into a plurality of wavelengths having a minute difference of a center wavelength. A wavelength of the reflected light is measured based on a logarithm of a ratio of a photocurrent by pairs of light-receiving elements (photodiodes (PD)) which are disposed at each of plural output channels of the AWG.
In this physical quantity measuring system, a wavelength range of the reflected light from the FBGs is assigned between two output channels which are located adjacently each other in the AWG.
In FIG. 5, dividers DIV1˜DIV5 output a ratio of a photocurrent of an adjacent photodiode by a logarithmic value.
This physical quantity measuring system has advantages that there is no movable portion, and that it is excellent in quake resistance and high-speed wavelength detectivity.
However, in this physical quantity measuring system, a reflected center wavelength of the FBG needs to be assigned between center wavelengths of two adjacent output channels in the AWG and in a linear part with respect to a relation between a wavelength change of a reflected light of the FBG and a logarithm value as shown in FIG. 6. Thereby, several problems will occur.
First of all, when the reflected center wavelength of the FBG changes more than a center wavelength interval (divisional wavelength bandwidth) of adjacent output channels in the AWG, it has been difficult to detect the change. That is, when an AWG having a broad divisional wavelength bandwidth is used in order to detect a great change of the reflected center wavelength in the FBG, a change of a logarithmic value becomes gentle (gradual) compared to that of the wavelength itself, and it becomes difficult to detect a minute change of the reflected center wavelength in the FBG. For example, an AWG which has a divisional wavelength bandwidth of 0.2, 0.4, 0.6, 0.8, or 1.6 nm, is currently marketed (e.g., Internet website of NTT Electronics Corporation as of Nov. 3, 2006, http://www.nel-world.com/products/photonics/awg_mul_d.html). Meanwhile, since the FBG has a strain sensitivity of about 1.2 pm/μstrain and a temperature sensitivity of about 10 pm/° C., when measuring a strain more than 1400 μstrain, the reflected center wavelength of the FBG changes more than 1.6 nm. Therefore, it is difficult to measure the strain including a change of the reflected center wavelength of the above commercial FBG.
Further, in order to assign the linear part with respect to a relation between a change of the reflected center wavelength and a logarithm value within a range of the reflected center wavelength of the FBG, an exact center wavelength design with respect to the AWG and the FBG is necessary.
Additionally, when the reflected center wavelength is changed more than the divisional wavelength bandwidth of the AWG due to a temperature or a strain, the reflected center wavelength of the FBG needs to be adjusted by changing a strain or a temperature of the AWG so that the reflected center wavelength of the FBG can be in the linear part with respect to a relation between a change of the reflected center wavelength and a logarithmic value.
The above problems will be described below in more detail.
When receiving a reflected light of the FBG, in which a reflected center wavelength thereof changes in a broad range covering from a divisional wavelength band of a first output channel of the AWG to a divisional wavelength band of another output channel adjacent to the first output channel (i.e., when receiving a reflected light from the FBG in case that a reflected center wavelength thereof slightly changes in a vicinity of a certain output channel of the AWG), since a transmission loss of the output channel adjacent to the former output channel is great, it is difficult to exactly measure an amount of a reflected light from the FBG.
For example, FIG. 7 shows a transmission characteristic of an AWG, which has a center wavelength interval (divisional wavelength bandwidth) of 1.6 nm (interval of 200 GHz in frequency), a half bandwidth of 0.8 nm, a maximum transmission rate of 100%, a transmission rate having a Gaussian distribution, and 4 output channels in a wavelength band of 1539˜1543.8 nm.
With respect to a center wavelength (λc) of each output channel, a transmission loss of an output channel #1 and an output channel #3, which are adjacent to an output channel #2, are 48 dB at the center wavelength of the output channel #2, which is extremely great. When a reflected center wavelength of the FBG moves to the center wavelength of the output channel #2, an amount of reflected light can be measured in the output channel #2, but it is difficult to measure the amount of the reflected light in the output channel #1 and the output channel #3, which are adjacent to the output channel #2, due to a great loss.
Next, a reflection loss characteristic of a fiber Bragg grating with respect to a wavelength is considered.
FIG. 8 shows a reflection loss characteristic with regard to a wavelength of an FBG, which has a half bandwidth of 0.1 nm, a maximum transmission rate of 90%, and a Gaussian distribution concerning the reflection loss characteristic with regard to a wavelength. FIG. 9 shows a characteristic of an amount of a light output with regard to a wavelength, which represents an amount of a light output with regard to a change of a reflected center wavelength that changes from 1539.4 nm to 1543.4 nm. In this case, a light amount as to a center wavelength of each output channel is defined as 1 (a ratio of the light amount is 0 dB).
As shown in FIG. 9, when an amount of a light output at a center wavelength of 1540.6 nm of the output channel #2 is set to 0 dB, an amount of light output of the output channel #1 and the output channel #3 as to the center wavelength of 1540.6 nm, is about 2*10−5 (about −47 dB) and very small. Since it is difficult to measure a light amount less than 1*10−4 (about −40 dB), as mentioned below, the amount of the light output of only output channel #2 can be measured in a wavelength range of 1540.46 nm to 1540.74 nm, which is shown by arrows in FIG. 9.
In this case, since a characteristic of an amount of a light output as to the channel #2 has a convex shape in which a center thereof is a center wavelength, when a reflected center wavelength of the FBG shifts from the center wavelength either to a shorter wavelength or to a greater wavelength, the amount of light output decreases. However, when the reflected center wavelength of the FBG changes slightly in a range of a wavelength of 1540.46 nm to 1540.74 nm which is in a vicinity of the center wavelength of the output channel #2, since the amount of light output is measured only as to the output channel #2, it is difficult to detect exactly whether the reflected center wavelength of the FBG changes to a shorter wavelength or to a greater wavelength.
Further, a reason why it is difficult to measure a light amount less than −40 dB is described below.
That is, it is difficult to obtain a crosstalk less than −40 dB in manufacturing of an AWG, and there is no filtering characteristic such as a Gaussian distribution in a wavelength range less than −40 dB.
Further, when a light of an output channel of an AWG is converted to electricity (analog) and then a voltage of the electricity is obtained in a digital value, it is difficult to measure a dynamic range more than 40 dB (the dynamic range more than 40 dB means, for example, to measure a voltage of 1 mV exactly and linearly when a maximum voltage is 10 V).